1. Field of the Invention
The present invention relates to a communications system, and particularly but not exclusively to channel management procedure for use when transmitting low rate data over a TDMA communications system such as a digital cordless telephone system for example DECT (Digital European Cordless Telephone). By low rate data is meant a data rate which is a binary submultiple of the data rate for one TDMA time slot.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings illustrate respectively an example of a digital cordless telephone system and the channel and message structure in accordance with the DECT protocol.
The illustrated digital cordless telephone system comprises a plurality of primary or base stations PS of which four, PS1, PS2, PS3 and PS4, are shown. Each of the primary stations is connected by way of a respective wideband landline link 10, 11, 12 and 13, capable of carrying data at a rate of say 1.152 Mbits/sec. to cordless telephone system controllers 14 and 15. The system control terminals 14 and 15 are, in the illustrated embodiment, connected to the PSTN which is constituted by an ISDN (Integrated Services Digital Network) link.
The system further comprises a large plurality of secondary stations SS some of which, SS1, SS2, SS4 and SS5, are hand portable and are used for digital time division duplex speech communication only. Others, for example SS3 and SS6, are data terminals which also are capable of duplex data communication. Duplex communication between the secondary stations within an area covered by a system control terminals and/or the PSTN is by way of radio through the primary stations PS, which act as relay stations. Accordingly the primary and secondary stations each comprise a radio transmitter and receiver.
Referring to FIG. 2, the illustrated system has ten radio channels, hereinafter referred to as frequency channels C1 to C10, each capable of carrying digitised speech or data at 1.152 Mbits/sec. The adjacent frequency channel separation is 1.728 Hz. Each frequency channel is divided in the time domain into 10 ms frames. Each frame is divided into 24 time slots (or physical channels) of which the first twelve F1 to F12 are allocated for transmission in a forward direction, that is from a primary station to a secondary station, and the second twelve R1 to R12 are allocated for transmission in the reverse direction. The forward and reverse time slots are twirled, that is, the correspondingly numbered forward and reverse time slots, for example F4, R4, comprise a twin which hereinafter will be referred to as a duplex voice channel. In setting-up a call between a primary and a secondary station, a duplex voice channel is assigned to the transaction. The assignment of the duplex voice channel in any of the frequency channels C1 to C10 is by the method of dynamic channel allocation, whereby a secondary station, taking account of its radio environment, negotiates with the primary station for access to the best duplex voice channel currently available.
The general structure of a message is also shown in FIG. 2. The message structure comprises two bytes of preamble 16, two bytes of a synchronisation sequence 18, six bytes of signalling data plus 2 bytes for cyclic redundancy check (CRC) 20 and forty bytes of digitised speech or data plus a four bit CRC (repeated twice) 22. The digitisation rate and data rate is 32 kbits/sec. Both the primary and secondary stations include a buffer to compress the 32 kbits/sec. data to bursts of data at 1.152 Mbits/sec. so that it is suitable for transmission.
The basic protocol for a transmission which is to be initiated by a secondary station SS is for it to listen to all the forward physical channels in each of the frequency channels C1 to C10 and ascertain which forward physical channels are busy and idle and the relative signal quality in these forward physical channels, and from the information derived the secondary station determines what it believes is the best duplex voice channel and transmits in the reverse physical channel of that duplex voice channel to a particular primary station PS. The signalling details 20 in the message together with the details 22 in the initial transmission are decoded and passed to the system controller 14 or 15 which sets-up the fixed network connection. The primary station confirms that the particular physical channel has been assigned to the transaction.
In the forward direction, the primary stations send paging messages to the addressed secondary stations in say every sixteenth frame. Such an arrangement enables the secondary stations to "sleep" during at least the intervening fifteen frames thereby economising on power. An addressed secondary station in response to a paging request addressed to it will, unless a duplex voice channel has been assigned, transmit in the reverse time slot (or physical channel) of the best duplex voice channel. As a general rule the system protocol will give priority to speech over data.
It is not unusual for a secondary station SS3 or SS6 to generate batches of data at rates in excess of 32 kbits/sec. Also, if the system is to be able to utilise an ISDN fixed wired link, then unless buffering is used, the system must be able to supply data at a rate of 144 kbits/sec. One way of doing this is for a system control terminal to allocate additional duplex voice channels to the transaction so that data packets can be transmitted in parallel.
There is also the alternative situation of a data terminal generating data at a rate which is a binary submultiple of 32 kbits/sec, for example 16 kbits/sec, and 8 kbits/sec, 4 kbits/sec. or 2 kbits/sec, such a terminal will hereinafter referred to as a low rate data terminal. If a physical channel in each frame is assigned to a transmission from a low rate data terminal this would be an inefficient use of the radio spectrum. It has been proposed to split a physical channel into 2 or more partial physical channels and assign each partial physical channel to a respective low rate data terminal. However in a situation of the signals from two or more low rate data terminals being multiplexed on one physical channel, and one of the data terminals completing its transaction before another low rate data terminal sharing the same physical channel, then the system control terminal either has to find another low rate data terminal having a data rate not exceeding the capacity of the released partial physical channel and multiplex that with the transaction from the subsisting low rate data terminal on that physical channel or pad the released partial physical channel with idle bits in order to maintain bit synchronisation.